Magnetic methods and apparatus for manipulating a beam of charged particles



Aprii 12, 1966 s. F. SKALA 3,246,147 MAGNETIC METHODS AND APPARATUS FORMANIPULATING A BEAM 0F CHARGED PARTICLES Filed Nov. 29, 1965 2Sheets-Sheet 1 Z mus B 'mv UN\FORM MAGNETIC FIELD -e Xmas X AXlS Y mus(mus INVENTOE 23 SF. kALA ATTO NEY April 12, 1966 s. F. SKALA 3,246,147MAGNETIC METHODS AND APPARATUS FOR MANIPULATING A BEAM OF CHARGEDPARTICLES Filed NOV. 29, 1963 2 Sheets-Sheet 2 United States PatentMAGNETIC METHODS AND APPARATUS FOR MANIPULATING A BEAM OF CHARGEDPARTICLES Stephen F. Skala, Chicago, Ill., assignor to Western ElectricCompany, Incorporated, New York, N.Y-, a corporation of New York FiledNov. 29, 1963, Ser. No. 326,827 Claims. (Cl. 250-495) The presentinvention relates generally to methods of virtually extending at leastone dimension of the effective cross section of a collimated beam ofcharged partices, and more particularly to methods and apparatus foruniformally irradiating troublesome product geometries. The generalobjects of the invention are to provide new and improved methods andapparatus of such character.

Penetrating radiation has, within the past few years, drawn widespreadindustrial attention as a processing tool. Modern applications ofpenetrating radiation include the preservation of food, thesterilization of drugs and medical supplies, the cross-linking ofplastics to improve their characteristics, the polymerization ofmonomers to yield polymers with unique qualities, and the initiation ofchemical reactions such as catalysis, isomerisation, halogenation andoxidation. The expanding horizons in the industrial utilization ofradiation correspondingly increase the demands for more versatiletechniques and devices for reducing radiation to a workable tool. 1

A significant problem area related to these demands is the manipulationof charged particles in a beam to increase the effective cross sectionthereof so that irradiation of substantial areas is facilitated. Manytechniques, most of which involve static fields, have been devised foraccomplishing this function, but all known techniques have significantlimitations. For example, static fields utilized to enlarge or shape acollimated beam of charged particles must affect some charged particleswithin an instantaneous cross section to alter their course more thanothers, in order to obtain the varying degrees of angularity requisitefor the desired shaping or enlarging of the beam. Even assuming auniform particle density over a cross section of the initial beam, theresulting cross section of the shaped or enlarged beam invariably has anonuniform particle density which gives rise to irregularities in thedose distribution and severely impairs the efiiciency of irradiation.

Uninhibited, a collimated beam of charged particles inherently has anonuniform particle density, the densityat the center being much greaterthan that at the peripheral portions of the beam. The most widely usedtechnique of enlarging the area of a beam and one which to a limitedextent circumvents this problem, is a process termed scanning. Thistechnique involves magnetically oscillating the beam laterally of itselfand longitudinally along an opening in a scanning horn to sweep the'beamangularly and momentarily to the normally lowerdensity peripheral areasof the horn opening so that the resulting b am is of relatively uniformradiation density.

It is of course obvious that because the beam oscillates laterally ofitself along the horn opening, the initial concentrated beam must bebent through continuously varying degrees of angularity to cover theentire length of the horn opening during a full sweep. To accomplishthis, the strength as well as the position of the magnetic field must becontinuously varied with precise accuracy.

In addition, since the concentrated beam occupies only a' small portionof the horn opening at any instant of time, the greater the degree ofangular displacement of the concentrated beam, the greater the degree ofspreading thereof over a portion of the'horn opening, and nonuniformparticle density again results; Further, since the from maximumefiiciency and 3,246,147 Patented Apr. 12, 1966 particles at theperipheral areas of the horn are angularly projected against an objectwith respect to those at the center, the depth of penetration andtherefore the radiation dosage is nonuniform over elongated areas tracedacross the object.

Therefore, it is an object of the present invention to provide a new andimproved technique for virtually extending at least one dimension of theeffective cross section of a collimated beam of charged particles,wherein the particles are bent through a constant angle and the strengthof the magnetic field remains constant, with the result that theradiation density of the beam remains substantially unaltered during thevirtual enlargement of the beam. Another object of the invention is toprovide new and improved methods of such character, wherein the particlepaths remain substantially parallel across the virtually elongated areaso that spreading of the beam is uniform thereacross and a uniformdosage of radiation results; and wherein because the angulardisplacement of the beam is constant and the spreading uniform, onedimension of the beam cross section may thus be elongated practicallywithout limit.

Still another object of the invention is to provide new' and improvedmethods of virtually elongating the cross section of a beam of chargedparticles, wherein all of the particles within the virtually elongatedcross section of transverse movement of the product'relative to the beamso that the cross section of the beam may be virtually enlarged toencompass the entire product area to be;

irradiated.

A method accomplishing the above and other objects in accordance withthe invention includes the steps of bending a collimated beam of chargedparticles through a substantial angle relative to the initial beam byarranging a magnetic field in the path thereof, and moving the magneticfield along the initial beam longitudinally thereof to cause the bentportion of the beam to sweep over an elongated area. 'In the morespecific environment of irradiating a surface, the initial beam isprojected generally parallel to and spaced from such'surface, and isbent toward the surface by the sweeping magnetic field of fixed strengthso as to impinge on an elongated portion 'thereof.

The entire surface maybe irradiated by coordinating intermittent orcontinuous movement of the surface transversely of the-beam withreciprocal movement of the bent portion of the beam so that successiveportions (intended to include contiguous portions and overlappingportions) of the surface in turn are irradiated on successive strokes ofthe bent beam.

In relation to another aspect of the invention, the irradiation oftroublesome product geometries also constitutes'an acute problem in theutilization of penetrating radiation as a processing tool. One of themost perplexing product geometries encountered is a generallycylindrical cross section, such as elliptical, ovoidal, polygonal, orsimilar cross sections, or portions thereof. The difficulty inirradiating generally cylindrical objects stems minimum costconsiderations, which require that the maximum available beam energy beabsorbed by the object and that the radiation digiribution over theproduct area be as uniform as poss1 e.

In the case of cylindrical products this means that cylindrical surfaceat all points therearound for the same increment of time.

While this problem appears to be trivial when it is considered that thecylindrical surface could be rotated with respect to the source of thebeam, or vice versa, to effect the desired result, it is not alwaysconvenient to rotate the object or the source, and their size andsensitivity often preclude this technique. It is also impractical tohave a plurality of independent sources arranged about the circumferenceof the object, each directing a beam which is incident normal tosucceeding contiguous portions around the circumference.

Many other techniques have been employed to alleviate this problem. Onesuch techniquerequires the inversion of one-half of a beam so that theobject is irradiated from two sides, thereby necessitating that theobject be rotated. through an angle of only 180 to irradiate an entirecircumference thereof. As before, however, it is not always convenientand sometimes impractical to rotate either the Object or the source.

It has also been proposed that a single beam, having a widthsubstantially greater than the cylindrical object, be bent in increasingdegrees, proceeding from the center of the beam to the peripheralportions thereof, so as to impinge normal to the surface all around acircumference of the object. This latter technique requires a magneticfield having a very critical involute configuration so that the strengththereof varies to bend each particle path with the same radius ofcurvature, the center of curvature for each successive path beingdisposed along a tangent to the object at the point of path intersectiontherewith, to enable successive particle paths to intersect normal tosuccessive portions of the cylindrical surface. Another disadvantage ofsuch a technique is that uniform distribution of the incident particlesover the product area is invariably sacrificed to permit simultaneousirradiation of the entire outer surface. Also, the intensity of theincident radiation is nonuniform about the product as a result ofnonuniformity of path lengths and air absorption. Finally the size ofarticle that may be irradiated is limited from a practical standpoint,since the initial beam of radiation is limited as to the degree ofenlargement obtainable, and must be much wider than the objectirradiated.

It is therefore another object of the present invention to provide newand improved methods and apparatus for irradiating a curved surface sothat charged particles impinge substantially normal to the surface atall points along the curve thereof. Still another object of theinvention is to provide methods and apparatus of such character forirradiating a generally cylindrical surface, wherein a single beam mayvirtually be rotated about the article without moving either the beamsource or the article, and wherein such rotation virtually elongates andexpands the cross section of the beam without altering the uniformity ofthe radiation density thereof.

A further object of the invention is to provide methods and apparatus inaccordance with the preceding object, wherein an initial beam of chargedparticles is acted upon by a simple combination of magnetic fieldsadvantageously employed to direct the beam radially into the product allaround a circumference thereof. Still further objects of the inventionare to provide methods and apparatus of such character, wherein the beamenergy is uniformly distributed over a circumference of the productarea, where in the intensity of the radiation is uniform about theproduct, and wherein the size of product that may be irradiated ispractically without limit.

Additional objects of the invention are to provide new and improvedapparatus for irradiating cylindrical articles, wherein simple, magnetictools in everyday use are advantageously combined to direct a singlebeam of charged particles radially into the article all around acircumference of the article, wherein two pairs of magnets are combinedso that rotation of one pair effects rotation of 4 the other, andwherein the combined rotational move ment of both causes virtualrotation of the beam about the article to bombard the article radiallywith charged particles all around a circumferential portion thereof.

The above and other objects may be accomplished in accordance with theinvention by projecting a collimated beam of charged particles generallyparallel to a tangent to a curved surface to be irradiated and spacedfrom the curved surface, and bending the beam with a first magneticfield so that the beam follows the curve of the surface. The curvedportion of the beam is then bent further by a second magnetic field sothat the beam impinges on a portion of the curved surface. The secondmagnetic field is moved along the curved beam so that the particlestherein are continuously directed into the moving second magnetic fieldand are deflected thereby into all portions of the surface which thecurved beam overlies.

Apparatus in accordance with a preferred embodiment of the invention forirradiating a generally cylindrical object includes a first pair ofmagnets which may be arranged concentrically about the object and spacedapart in opposed parallel relationship. The first magnets produce auniform magnetic field therebetween which is circumferentiallycontinuous about the object, and which is arranged relative to a beam ofcharged particles so that the beam enters the field transversely,between the first magnets, and tangentially, beyond the outer surface ofthe object.

The strength and polarity of this first magnetic field is selected toexert a force on the charged particles which causes the particles in thebeam to orbit the object. A second pair of magnets, each of which iscarried by one of the first magnetic sources, is arranged to create asecond magnetic field along a portion of the orbital path of theparticles designed to deflect the orbiting particles radially into theobject. Mechanism is provided for rotating the first magnets, thereby torotate the second magnetic field about the object so that the orbitingparticles, directed into the moving second magnetic field, are deflectedradially into all portions of the surface which the orbital pathoverlies.

Other objects, advantages and aspects of the invention .will appear fromthe following detailed description of a preferred embodiment thereofwhen taken in conjunction with the appended drawings in which:

FIG. 1 is a diagrammatic illustration of the interaction of an electronwith a magnetic field in accordance with an elementary physicalprinciple underlying the invention;

FIG. 2 is a diagrammatic illustration of the interaction of an electronwith a magnetic field exemplary of a second and equally elementaryphysical principle underlying the invention;

FIG. 3 is a perspective view depicting a preferred apparatus embodyingthe invention in a refined form;

FIG. 4 is an elevational cross section of the preferred embodiment inFIG. 3, taken generally along the center line of the apparatus of FIG.1;

FIG. 5 is a front elevational view, partly in section, of a specificembodiment illustrating the invention in a broader context;

FIG. 6 is a front elevational view, partly in section, of the apparatusin FIG. 5, exhibiting the effect of displace ment of a magnetic fieldalong a beam of radiation; and

FIG. 7 is a side elevational view, partly in section, of the apparatusdepicted in FIGS. 5 and 6, illustrating further the relationship of themagnetic sources, the initial beam of radiation and the article to beirradiated.

As stated at the outset, penetrative radiation has been found to havemany applications in industrial processing. Of particular interest in apreferred embodiment of the invention is the irradiation of plastics,such as polyethylene, to improve such characteristics as strength,temperature resistance, and insulating properties of the plastics. Thechemical mechanism accomplishing the improvement of such characteristicsas .a result of irradiation is a phenomenon termed cross-linking.

Exposure of a plastic material to penetrative radiation causes ejectionof an individual hydrogen atom from various carbon chains. The vacatedbonds on separate carbon chains subsequently move relative to each otherand eventially combine effectively to cross-link the chains. Because ofthe attendant advantages of crosslinked plastics, such technique hasgreat utility in improving the characteristics of plastic insulation onwires or cables, particularly in the telephone industries where theliterally hundreds of thousands of miles of wire manuf actured each daymust meet extremely stringent quality requirements.

The problem that immediately arises and one which is well recognized bythose skilled in the art, is the difiiculty in irradiating certaintroublesome product geometries, such as the cylindrical cross section ofa wire or cable. The radiation must uniformly impinge normal to thesurface at all points around the circumference thereof in order toobtain optimum depth of penetration and a uniform distribution of beamenergy over the product surface. Since such criteria are determinativeof irradiation efiiciency, and since the utilization of radiation as aprocessing tool is still an expensive proposition, the necescity ofmeeting the above criteria is apparent.

The present invention, in a refined context, is directed to methods andapparatus for irradiating such troublesome product geometries byorbiting charged particles of a collimated beam about the product anddirecting the orbiting particles radially into such product from allpoints along the orbit.

The fundamental concepts embodied in the present invention are groundedin two basic principles of physics. Considering the first of theseprinciples with reference to the three dimension-a1 coordinate system inFIG. 1, an electron -e traveling at a velocity v along the X axisperpendicular to a magnetic field having a flux density B along the Zaxis, is subjected to a force F in a direction perpendicular to a planecontaining the magnetic field and the direction of electron travel, andtherefore along the Y axis. This phenomenon is governed by the vectorialequation:

F=-eii x E which, in this special case becomes:

/ F levB/ The second basic principle relied upon, illustrated in FIG. 2,is that an electron -e traveling in the plane of the .X and Y axes witha velocity v perpendicular to a uniform magnetic field which has a fluxdensity B along the Z axis, can be maintained in an orbit defining acircle of radius r if a continuing force F is exerted on the electron bythe magnetic field which equals the centrifugal force acting upon theelectron in the orbital path. Expressed as an equation, thisrelationship becomes:

in which in is the realistic mass of the electron because the speed ofelectrons used for irradiation is significant compared to the speed oflight. Since the mass of the electron then becomes the sum of the restmass and the kinetic energy which equal, respectively, 0.51 m.e.v.s andfrom 0.5 to 1 m.e.v.-s, the mass will vary between 0.51+O.5=1 and0.51+1=1.5 m.e.v.s.

Referring now to the preferred embodiment illustrated in FIG. 3, thepolyethylene jacket on a cable 11 is to be irradiated, the cable havinga generally cylindrical geometry. The cable is axially passed throughthe central opening 12 in each of a pair of annular permanent magnets1313 which are spaced apart in opposed, parallel relationship. A hollow,axially elongated, annular envelope 14 is mounted intermedite theannular magnets 1313 so as to permit rotational movement of the magnetsrelative thereto. The inner and outer walls 15 and 16, respectively, ofthe envelope 14 are closed at the ends to define a chamber radiallycoextensive with the annular magnet-s 1313 and the chamber is evacuatedto provide a high vacuum therewithin.

The outer wall 16 and the closing ends of envelope 14 are preferablyformed of a material suitable to absorb the particular type of radiationemployed, yet pervious to magnetic fields, such as glass in the instantcase. The outer wall 16 is preferably provided with an integral tube 17(FIG. 4) extending transversely and tangentially of the envelope 14, andcommunicating with and interconnecting the evacuated chamber and anelectron accelerator 18 to define an evacuated path for a beam 19 ofaccelerated electrons. The inner wall 15, however, must be formed of athin foil which is pervious to the radiation because it constitutes anexit window for the radiation. It has been found that foils such as 2mils of titanium, or 5 mils of aluminum, or 7 mils of stainless steelare transparent to accelerated electrons and yet can withstand thevacuum required.

The permanent magnets 1313 are designed to produce therebetween auniform magnetic field which is circumferentially continuous about thecable 11 and which permeates the evacuated chamber within the envelope14. The strength and polarity of the uniform magnetic field are selectedto exert a force on the electrons in the beam 19, in accordance with thesecond physical principle discussed above, which substantially countersthe centrifugal force acting upon such particles in a circular orbitconcentric within the envelope 14. Since the entire orbital path lieswithin the magnetic field and since the magnetic field iscircumferentially uniform within the glass envelope 14, the electrons inthe beam will be deflected into and maintained in the orbital pathdefined by the envelope 14.

The orbiting electrons are deflected radially into the cable by a secondmagnetic field created between two permanent magnets 2121 each of whichis fixedly incorporated in one of the annular permanent magnets 1313 (asfor example in a bore the-rein) in opposed, aligned relationship withthe other. The size of the permanent magnets 21 21 is selected so thatthe magnetic field created therebetween occupies only a smallcircumferential portion of the chamber defined by the envelope 14. Thestrength and the polarity of the second magnetic field are selected toexert a force upon the orbiting electrons, in accordance with the firstphysical principle discussed above, which deflects the orbitingelectrons radially into the cable. Hence, the electrons in the beam 18enter the uniform magnetic field and travel in an orbital path about thecable 11 until they enter the second magnetic field where they aredeflected radially into the cable.

A ring gear22 is rigidly mounted about each of the annular permanentmagnets 13-13 (FIG. 3) in meshing relationship with a driving gear 23fixed to a shaft 24 driven at a constant speed in the directionindicated by the arrow A. Rotation of the gears 23 on the shaft 24causes rotation of the combined magnetic unit (designated generally bythe numeral 26 and consisting of the annular permanent magnets 13-13 andthe permanent magnets 21-21) about the central axis of the annularmagnets in the direction indicated by the arrow B. Rotation of theannular magnets in no way affects the first magnetic field because ofthe circumferential uniformity and the symmetry thereof.

Rotation of the unit 26 in turn results in rotation of the secondmagnetic field about the cable along a path substantially coincidentwith the orbital path of the electrons. As the second magnetic fieldrotates about the cable, the

electrons follow along therebehind in their orbital path until theyenter the moving second magnetic field and are deflected radially intothe cable. Therefore, upon rotation of the second magnetic field throughone complete revolution, all portions of the cable which the orbitalpath of the electrons overlies are subjected to radial bombardment byelectrons.

In order to effect irradiation of the entire length of cable, the cablemay be displaced intermittently or at a constant speed (preferably thelatter) longitudinally of the unit 26, in the direction indicated by thearrow C, by any conventional device, such as a capstan. The 1ongitudinalmovement of the cable and the constant speed rotational movement of theunit 26 may be coordinated to irradiate overlapping circumferentialportions (intended to include axially contiguous circumferentialportions as well) of the cable on successive revolutions of the secondmagnetic field in such a manner that substantially the entire length ofcable receives a uniform dose of radiation. In the case of constantspeed displacement of the cable, the resulting path of the radiallyincident radiation relative to the cable defines a helix. Optimumuniformity of dose distribution is achieved in such case by havingsuccessive convolutions of the helical path about the cable overlap asubstantial amount.

In the specific application contemplated, it is most convenient toirradiate the cable immediately after the polyethylene jacket has beenextruded therearound, thereby eliminating the additional handling whichwould be required in an irradiation operation at a remote location. Theopenings in the annular magnets 1313 and the envelope 14, are thereforemade larger by a substantial degree than the outer dimension of thecable so that no frictional engagement results therebetween.

In the above embodiment, therefore, the present invention advantageouslyaccomplishes virtual rotation of the beam about the cable. Therevolution of the magnets 21 and the resultant rotation of the bentportion of the beam, virtually elongates the cross section of the beamwhile the actual size of the beam remains unchanged and the uniformparticle density thereof is maintained. Also, since all particles in thebeam are bent through the same angle, beam spreading is uniform.Further, uniform distribution of the incident particles over the productarea is effected since all portions around the surface are exposed toall portions of the rotating beam for the same increment of time.

Finally, the intensity of the incident radiation is substantiallyuniform all around the circumference of the cable. This becomes apparentwhen it is considered that beam attenuation arises from an interferingmedia such as the atmosphere. Since substantially all interfering mediais evacuated from the envelope, absorption is completely eliminated andthe beam intensity is substantially uniform for all orbiting particlesdeflected into the cable. It is apparent, therefore, that the variationsin particle path lengths within the glass envelope is of negligibleconsequence insofar as it affects beam intensity.

The preferred technique of irradiating troublesome product geometriesthus evolved, includes bending the initial beam of radiation with acircumferentially uniform magnetic field so that the particles in thebeam orbit the product, deflecting the orbiting particles radially intothe product with a second magnetic field disposed along a portion of theorbital path, and moving the second mag netic field along a pathsubstantially coincident with the orbital path so that all portions ofthe surface which the orbital path overlies may be irradiated.

It is to be noted that a radially uniform and constant first magneticfield may not be desirable; that is, it might be desirable to providefor a certain degree of flexibility with respect to the velocity of theaccelerated particles. Particles with varying velocities entering aradially uniform and constant magnetic field designed to orbit particlestraveling at a particular velocity, might very well spiral out of theintended orbit. To accommodate such variation in particle velocities, itis suggested that the strength of the magnetic field be increased atprogressives ly greater radii. A certain minimum strength may beselected for the inner radius of the magnetic field to orbit particleshaving a lower limit of the particle velocities, and a certain maximumstrength may be selected for the outer radius of the magnetic field toorbit particles hav-. ing an upper limit of the particle velocities, thefield strength at intermediate radii being uniformly incrementaltherebetween. The field strength must, however, be circumferentiallyuniform and constant at all radii of the field. With such radiallyincreasing magnetic field strength, all particles with velocitiesbetween the upper and lower limits will be urged toward an orbital pathof a mean radius.

Turning now to a description of the invention in a broader context, theobject to be irradiated may have other troublesome geometries, evennonsymmetrical ge-. ometries. The same technique may beemployed;.however, in this instance the beam is initially directed alonga path substantially conforming to the contour of and spaced from thesurface to be irradiated, which path coincides with at least one surfacedimension. The beam is deflected from such path in a direction inwardlytoward the surface, and the point of deflection is simultaneously causedto move, at least intermittently, along such path so that the beamsweeps over an elongated area along the chosen surface dimension.

Or, the object may have a simple plane surface which is to beirradiated, such as the article 36 in FIGS. 5, 6 and 7. In thisembodiment, only a single magnetic field is required and the beam isprojected parallel to and spaced from the surface to be irradiated. Themagnetic field is produced between a pair of permanent magnets 3131(FIG. 7) spaced apart in opposed, parallel relationship on oppositesides of an evacuated envelope 32 having an exit window 33 of suitablefoil material. The envelope 32, as in the preferred embodiment,communicates with an electron accelerator 34 to provide an evacuatedpath for a collimated beam 35 of accelerated electrons. The magnets 3131are mounted at the upper ends of opposite legs of a yoke 36; so as to bedisposed on opposite sides of the beam, as shown in FIG, 7, and the yokeis slidably mounted on a pair of guide rods 37-- 37 for movement alongthe beam longitudinally thereof. This movement may be effected byapparatus such as the cylinder and piston arrangement 38 shown in FIGS.5 and 6.

The strength and polarity of the magnetic field is selected to exert aforce on the electrons in the beam, in accordance with the firstrelationship disclosed above, to deflect the particles through aprescribed angle of substantial magnitude, preferably toward the surfaceto be irradiated. Displacement of the yoke 36 in the direction of thearrow D so that the magnetic field traverses the surface of the article30 at a constant speed (from the left extreme shown in FIG. 5 to theright extreme shown in FIG. 6), effects uniform distribution of the beamenergy over the portions of the surface which the beam overlies. Sincethe beam impinges at right angles to the surface, optimum depth ofpenetration is also obtained.

In the case of small products to be irradiated, such as that depicted inFIGS. 5-7, the article may be moved transversely of the beam, by asimple mechanical device, so that the entire surface of the object maybe irradiated. Uniform distribution of beam energy over the product areamay be maintained by coordinating intermittent movement of the producttransversely of the initial beam with reciprocal movement of themagnetic field longitudinally along the beam, so that contiguous oroverlapping portions of the surface are irradiated on each successivestroke of the field across the surface.

In the case of large products, however, it may be desirable to invertthe yoke with respect to FIGS. 5 through 7, so that it lies entirely onthe same side of the product as the beam, in which case the movement ofa continuous surface may be coordinated with the reciprocal movement ofthe magnetic field in the above manner to irradiate the entire surface.

Where uniform distribution of beam energy over the product surface is animportant factor, the movement of the product in either of the abovecases should be intermittent. if uniform distribution is not important,product movement may be continuous. In the latter case, overlappingportions of the product surface will be irradiated on successive strokesof the second magnetic field, and the degree of overlap will vary withineach stroke.

The unique result obtained by all the above techniques is that the areaof a collimated beam of radiation may be virtually elongatedsubstantially without limit by bending the beam with a magnetic fieldand moving the magnetic field longitudinally along the beam. The beammay be virtually enlarged to encompass an entire surface of the articleby coordinating intermittent or continuous longitudinal movement of thearticle with movement of a magnetic field.

While a preferred embodiment is described in detail hereinabove and amore general description is provided of the invention in a broadercontext, various modifications may be made without departing from thespirit and scope of the invention and it is intended that all suchmodifications be interpreted as contemplated by the invention.

What is claimed is: -1. A method of extending at least one dimension ofthe effective cross section of a collimated beam of charged particles,which comprises the steps of:

bending the beam through a prescribed angle of substantial magnituderelative to the initial path of the beam by arranging a magnetic fieldin said path; and

moving the magnetic field along the initial path of the beamlongitudinally thereof to cause the bent portion of the beam to sweepover an elongated area.

2. A method of irradiating portions of a surface of an article, whichcomprises the steps of:

projecting a collimated beam of charged particles generally parallel toand spaced from the surface to be irradiated;

bending the beam by arranging a magnetic field in the path thereof,through -a prescribed angle relative to the initial path of the beam andtoward said surface so that the bent portion of the beam impinges on aportion of said surface; and

moving the magnetic field along the initial path of the beamlongitudinally thereof, to cause the bent portion of the beam totraverse at least one dimension of said surface.

3. A method of irradiating portions of a surface of an article, whichcomprises the steps of:

projecting a collimated beam of charged particles generally parallel toand spaced from the surface to be irradiated;

bending the beam by arranging a magnetic field in the path thereof,through a prescribed angle relative to the initial path of the beam andtoward said surface so that the bent portion of the beam impinges on aportion of said surface;

moving the article parallel to said surface thereof and transversely ofthe initial path of the beam; and moving the magnetic field along theinitial beam longitudinally thereof;

whereby at least two dimensions of said surface may be traversed by thebent portion of the beam.

4. A method of irradiating a prescribed surface area of an article,which comprises the steps of:

projecting a collimated beam of charged particles generally parallel toand spaced from the surface area to be irradiated;

bending the beam by arranging a magnetic field in the path thereof,through a prescribed angle relative to the initial path of the beam andtoward said surface area so that the bent portion of the beam impingeson a portion of said surface;

moving the article to cause the bent portion of the beam to trace afirst path over the prescribed surface area;

moving the magnetic field along the initial path of the lbeamlongitudinally thereof to cause the bent portion to trace a second pathover the prescribed surface area generally perpendicular to the firstpath; and

coordinating the movements of the article and of the magnetic field sothat the movement of one is reciprocal across the correspondingdimension of the prescribed surface area at a rapid rate relative to themovement of the other, whereby the entire prescribed area may beirradiated.

5. A method of irradiating a prescribed surface area of an article,which comprises the steps of:

projecting a collimated beam of accelerated electrons generally parallelto and spaced from the surface area to be irradiated; bending the beamby arranging a magnetic field in the path thereof, through an angle ofsubstantially relative to the initial path of the beam and toward saidsurface area so that the bent portion impinges on a portion of saidsurface; moving the article generally parallel to said surface areathereof and transverse to the initial beam; and

reciprocably moving the magnetic field along the initial path of thebeam longitudinally thereof to traverse said surface area, thereciprocal movement of the magnetic field being coordinated with themovement of the article so that successive portions of the prescribedsurface area may be irradiated in turn on successive strokes of themagnetic field, whereby the entire prescribed surface area of thearticle may be irradiated. 6. A method of irradiating objects of complexshape and objects having large surface areas, with a single collimatedelectron beam, without appreciably affecting the initially establishedparticle density distribution across the cross-section of the collimatedbeam, which comprises the steps of:

forming and collimating a high energy electron beam; directing thecollimated beam along an initially established path closely adjacent toat least one surface dimension of an article to be irradiated; and

deflecting the beam at a substantial angle in a direction inwardlytoward the article to be irradiated by moving a magnetic field throughat least a plurality of points in succession along at least a portion ofthe initially established path, thereby causing the beam to irradiate atleast a portion of the article surface along the chosen dimensionthereof.

7. Apparatus for deflecting charged particles in a collimated beam intoan orbital path about an object and for deflecting the orbitingparticles into the portions of the object which the orbital pathoverlies, which comprises:

first magnetic means rotatable about its polar axis for producing afixed, continuous and uniform magnetic field about the object;

means for producing a collimated beam of charged partioles directed intothe first magnetic field transversely thereof and outwardly of theperiphery of the object, the strength and polarity of the first magneticfield being selected to exert a force on the charged particles whichsubstantially counters the centrifugal force acting upon such particlesin following a circumferential path about said axis of the object, sothat the particles may orbit the object with said axis defining thecenter of rotation;

second magnetic means carried by said first magnetic means atsubstantially the radius of the orbital path of the charged particles,for producing within the first magnetic field a second magnetic fieldhaving a 1 1 strength and polarity selected to deflect the particlesfrom their orbital path and into the object; and means for rotating saidfirs-t magnetic means about said axis, thereby to rotate the secondmagnetic field about said axis along a path generally coincident withthe orbital path of the particles;

whereby the orbiting particles are deflected into all portions of theobject which the orbital path of the particles overlies.

8. The apparatus as recited in claim 7, wherein: an evacuated envelopeis disposed within the magnetic field produced by the first magneticmeans and is continuous about the object, to enclose the initial beampathand the orbital path of the charged particles and to provide a lowdensity medium of travel therefor so that beam intensity may besubstantially uniform all about the object regardless of unequaldistances of particle travel, said evacuated envelope beingsubstantially transparent to the magnetic fields.

9. Apparatus for irradiating a generally cylindrical object so that theradiation penetrates the outer surface of the object radially at allpoints around a circumference thereof;

a pair of annular magnets axially spaced apart in opposed, parallelrelationship and arranged to permit the object to pass axiallytherethrough, said annular magnets producing a magnetic fieldtherebetween which is circumferentially continuous and uniform aboutsuch objects;

means for producing a collimated beam of charged particles directed intothe first magnetic field transversely and tangentially thereof at aradius beyond the outer surface of the object, the strength and polarityof the first magnetic field being selected to exert a force on thecharged particles which substantially counters a centrifugal forceacting upon such particles in following a circumferential path about theobject so that the particles may orbit the object;

a second pair of magnets, each fixedly incorporated;

in a portion of one of said annular magnets at substantially the radiusof the orbital path of the particles, said second pair of magnets beingaligned parallel to the axis of said annular magnets; said secondmagnets producing a magnetic field thereb-etween having a strength andpolarity selected to 12 exert a force on the orbiting particles whichdeflects them from their orbital path radially into the object so as topenetrate the portions of the surface thereof which the intersection ofthe orbital path and the second magnetic field overlies; and

' means for rotating said annular magnets in synchronism about the axisthereof, thereby to rotate the second magnetic field about the objectalong a path generally coincident with the orbital path of theparticles;

whereby the charged particles are deflected by the second magnetic fieldradially into all portions of the core which the orbital path of theparticles overlies.

10. The apparatus as recited in claim 9, wherein:

the annular magnets comprise a pair of annular permanent magnets;

an evacuated envelope having a central opening therethrough isinterposed between said annular permanent magnets with the centralopenings therein communicating with the central opening in saidenvelope, said envelope having inner and outer walls which are closed atthe ends and an integral tube communicating with and connected to thebeamproducing source to enclose completely the path of the initialcollimated beam and the orbital path of the charged particles in avacuum, the inner wall of said envelope being substantially transparentto the charged particles, and the outer wall and closing ends beingsubstantially transparent to the magnetic fields; and

the second pair of magnets are permanent magnets.

References Cited by the Examiner UNITED STATES PATENTS 2,741,704 4/1956Trump et a1 25049.5 2,858,441 10/1958 Gale 250-495 2,897,365 7/1959Dewey et a1 250-495 3,010,018 11/1961 Zitfer 25052 3,104,321 9/1963Smith 25049.5 3,109,931 11/1963 Knowlton et a1 25049.5

FOREIGN PATENTS 789,456 1/ 1958 Great Britain.

RALPH G. NILSON, Primary Examiner.

1. A METHOD OF EXTENDING AT LEAST ONE DIMENSION OF THE EFFECTIVE CROSSSECTION OF A COLLIMATED BEAM OF CHARGED PARTICLES, WHICH COMPRISES THESTEPS OF: BENDING THE BEAM THROUGH A PRESCRIBED ANGLE OF SUBSTANTIALMAGNITUDE RELATIVE TO THE INITIAL PATH OF THE BEAM BY ARRANGING AMAGNETIC FIELD IN SAID PATH; AND MOVING THE MAGNETIC FIELD ALONG THEINITIAL PATH OF THE BEAM LONGITUDINALLY THEREOF TO CAUSE THE BENTPORTION OF THE BEAM TO SWEEP OVER AN ELONGATED AREA.
 7. APPARATUS FORDEFLECTING CHARGED PARTICLES IN A COLLIMATED BEAM INTO AN ORBITAL PATHABOUT AN OBJECT AND FOR DEFLECTING THE ORBITING PARTICLES INTO THEPORTIONS OF THE OBJECT WHICH THE ORBITAL PATH OVERLIES, WHICH COMPRISES:FIRST MAGNETIC MEANS ROTATABLE ABOUT ITS POLAR AXIS FOR PRODUCING AFIXED, CONTINUOUS AND UNIFORM MAGNETIC FIELD ABOUT THE OBJECT; MEANS FORPRODUCING A COLLIMATED BEAM OF CHARGED PARTICLES DIRECTED INTO THE FIRSTMAGNETIC FIELD TRANSVERSELY THEREOF AND OUTWARDLY OF THE PERIPHERY OFTHE OBJECT, THE STRENGTH AND POLARITY OF THE FIRST MAGNETIC FIELD BEINGSELECTED TO EXERT A FORCE ON THE CHARGED